Update docker/setup-buildx-action action to v3

Created by: renovate[bot]

This PR contains the following updates:

Package	Type	Update	New value	References	Sourcegraph
docker/setup-buildx-action	action	major	v3	source

Test plan: CI should pass with updated dependencies. No review required: this is an automated dependency update PR.

Release Notes

Configuration

Schedule: Branch creation - "on the 1st through 7th day of the month" in timezone America/Los_Angeles, Automerge - At any time (no schedule defined).

Automerge: Disabled by config. Please merge this manually once you are satisfied.

Rebasing: Whenever PR becomes conflicted, or you tick the rebase/retry checkbox.

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